Microsphaera diffusa

Microsphaera diffusa is a plant pathogen that is known to cause powdery mildew on soybeans.^[1]

Microsphaera diffusa
Scientific classification
Kingdom:	Fungi
Division:	Ascomycota
Class:	Leotiomycetes
Order:	Erysiphales
Family:	Erysiphaceae
Genus:	Microsphaera
Species:	M. diffusa
Binomial name
Microsphaera diffusa Cooke & Peck
Synonyms
Erysiphe diffusa Erysiphe glycines Trichocladia diffusa

Importance

Powdery mildew is a significant pathogen of soybeans and tends to cause epidemics about every 10–15 years in Wisconsin. The first epidemic there was observed in 1975 and several have occurred since.^[2] When 82% of the soybean leaf area is covered by M. diffusa, photosynthetic and transpiration rates are less than half of normal soybeans, thus affecting soybean yield.^[3] Different studies have found varying amounts of yield reduction due to powdery mildew. In Illinois, measured yield losses ranged up to 14 percent.^[4] Studies from Iowa recorded yield losses of up to an estimated 10 bushels per acre. In Wisconsin, yield loss was up to 5 bushels per acre. It is important to note that yield loss due to powdery mildew will be greater for soybeans planted late for a region compared to early-planted soybeans.^[5]

Environment

Temperature plays an important role in powdery mildew development. Powdery mildew develops favorably in cooler temperatures (65–77 °F). Temperatures above 30 °C appear to constrain disease development. Rainfall does not appear to affect the disease,^[6] but it has been found that a shorter leaf wetness duration appears to be a driver of the disease.^[7] Additionally, low relative humidity is required for disease development.^[5]

Management

Variety selection is a tool that can be used to help combat powdery mildew. No variety of soybean has complete resistance to powdery mildew, but some varieties are clearly more susceptible than others. Resistance affects initial inoculation of the plant.

Fungicides may also be used as a management tool. They can be sprayed once powdery mildew is detected to kill the spores. This affects the dispersal and secondary inoculation of the plant.^[2] Some examples of fungicides include Topsin M, Quadris, and Headline, with the last two being less effective.^[5]

Another management practice is the planting date. Early-planted soybeans tend to show less severity of powdery mildew than late-planted soybeans.^[2]